Interpretive Summary: Drought is the biggest ‘profit stealer’ in U.S. soybean. Why is this so? Two simple facts tell the story: 1) drought occurs often in soybean production areas (somewhere every year), and 2) most U.S. soybeans are not irrigated (the states of MS and NE being notable exceptions). When drought strikes, farmers have few remedies at their disposal which can be brought to bear. For problem weeds, farmers can spray. For nematodes or phytophythora, farmers can choose resistant varieties. For drought, farmers just watch the crop suffer while dry weather steals away yield. In 1980, no one in the world could point to any soybean genotype and say, ‘this plant has agricultural drought tolerance’. The starting point in drought research was to search for soybean types which differed for seed yield and water response in the field under drought. USDA found several promising exotic types in the course of its research. This paper is a part of a series of investigations which are aimed at figuring out what makes the drought-tolerant types tick. An improved understanding of what we are dealing with will speed up the process of transferring drought tolerance to the farmer’s field.

Technical Abstract:
Three physiological traits that may affect performance of soybean (Glycine max (L.) Merr.) when soil water availability is limiting are i) water use efficiency (WUE), ii) regulation of whole plant water use in response to soil water content, and iii) leaf epidermal conductance (ge) when stomata are closed. Six soybean plant introductions (PIs), eight breeding lines derived from them, and nine cultivars were compared for variability in these three traits during vegetative growth in two greenhouse studies. In the first experiment, whole plant water use, normalized both to plant size and evaporative demand (the normalized transpiration ratio, NTR), was monitored during a 10-d cycle of gradually increasing drought stress, and then for an additional two days following rewatering. The critical soil water content at which each plant began to reduce its water use (FTSWC), was determined. The WUE was estimated as the ratio of total plant dry weight to total water used. In the second experiment, ge was determined for these same 23 genotypes by measuring leaf water vapour exchange after a 36-h dark adaptation. Substantial variation was found among genotypes for WUE, FTSWC, ge, and also the extent to which NTR recovered upon rewatering. Generally, adapted cultivars had greater WUE and lower ge than did PIs. However, PI 471938 and its progeny N98-7264 were clear exceptions to this trend. An unexpected finding was that WUE was significantly negatively correlated with ge across genotypes.